The invention starts out from a vacuum cleaner according to the preamble of the main claim. In a known vacuum cleaner of this type (German Disclosure Document No. 2 336 758) one has attempted to keep the air velocity constant in the area of the suction line or suction pipe and to provide in this manner the possibility to determine the quantity of dust, solid particles, and the like, drawn in by the vacuum cleaner, i.e. to determine the dust concentration. To this end, a light source and a photoelectric cell are provided in perpendicular arrangement relative to the air current, and the output signal of the photoelectric cell is used to influence the speed of the electric motor driving the blower of the vacuum cleaner via a corresponding control device.
Accordingly, this known vacuum cleaner proceeds from the realization that for controlling the operation or power of the vacuum cleaner it is an appropriate measure to evaluate that value which is actually influenced by the vacuum cleaner during its operation, namely the quantity of dust contained in the air. It is, therefore, a practical solution to control the power of the blower motor of a vacuum cleaner in response to the dust concentration. The before-mentioned publication does not, however, contain any indications as regards the type, design, function or possible implementations of the motor speed control operating in response to the dust concentration which is detected by optical means, and the attempt to keep the air flow constant in the measuring area of the suction pipe also does not help notably to arrive at a speed control that would suit practical needs.
Another critical point is seen in the only possibility mentioned for realizing the control, namely by means of a light source and a photoelectric cell. Such an arrangement permits, however, only to observe the dust conditions in the suction area along one single line. A real picture of the dust concentration actually encountered is not obtained so that the inaccuracy factor is correspondingly high. The publication does not contain any further information regarding the evaluation of the optically determined dust concentration. There has been further known a solution (German Disclosure Document No. 34 31 164), in which the light-optical unit comprising a luminous diode and a phototransistor is followed, after amplification of its output signals, by a switching element, i.e. a monoflop, which acts as a pulse shaper and at the same time as a delay element and which indicates the determined dust quantity by different optical means (green or red luminous diode) and can be re-triggered at any time after the end of its holding time when new dust is encountered. An evaluation block connected after the monoflop and reacting, accordingly, to the triggering signal for the luminous diodes permits the power of the blower motor of the vacuum cleaner to be controlled in response to the measured dust quantity.
This solution does not permit a continuous power control because the flip-flop action of the monoflop can be interpreted only as "up" or "down".
Now, it is the object of the present invention to improve the known vacuum cleaner in such a manner that in spite of a substantial simplification of the switching components forming the evaluation system, the power control of the blower motor will react exclusively to the dust quantity measured at any time and that, in particular, the speed is varied rapidly, instantaneously and infinitely.